2. Background of the Invention
Composite materials are well known. Fillers are usually added to composite materials, including composite polymers, to save cost or to enhance a particular mechanical property or other characteristic of the materials. The usage of fillers is usually accompanied by coupling agents that enhance the polymer-filter and filler-filler interaction so that the expected properties are realised.
The present invention is concerned with fillers which enhance the anti-static, flame retardant, accelerator, plasticiser, blowing characteristic and/or other physical or mechanical properties of composite materials and has particular application for use in composite polymers. Such have wide application in the aeronautical, mining, computer, road building, textile, foot ware, rubber and polyurethane industries among others. For example, it is often desirable to prevent the build up of static charges which can cause sparks (and hence explosions or electrical damage) or production problems, eg. collection of dust and poor feeding of materials through machinery. More highly conductive composite polymers can also be used for Electro Magnetic Interference shielding, for example.
Carbon black fillers, aluminium flakes and fibres, stainless steel fibres and chopped carbon fibres have all been used as fillers for the purpose of rendering composite plastic conductive. Likewise other chemicals such as Halogen compounds or triethyl phosphate have been used to achieve the flame retardant property.
A number of theories have been proposed to explain how discreet particle fillers impart conductivity and flame retardant properties in composite plastics.
In order for current to flow in a conductive polymer compound, electrons must travel along the filler as the plastic itself is an excellent insulator. To achieve this flow the discreet particles of the filler must be in contact or separate by a minimum distance which is probably less than 100 Angstroms. There are three properties of the filler particles which will effect the average inter-particle distance for a given filler loading in a polymer system. These are particle size, shape (structure), and porosity. Smaller size, irregular shape and high porosity all result in smaller inter-particle distances and hence higher conductivity. A fourth property of the particle which is relevant to conductivity and flame retardant properties in the composite plastics is surface chemistry, that is the presence of oxygen on the surface. The presence of appreciable quantities of oxygen on the surface (called volatile content) acts as insulation and hence reduces conductivity.
The known conductive fillers such as carbon black, aluminium, stainless steel and carbon fibres are expensive and furthermore some of these materials have other processing difficulties, eg. aluminium fibres and stainless steel fibres settle in liquid environments due to their high density. Further problems with known conductive fillers are that they often compromise other properties of composite plastics such as flame retardance and strength.
Static electrification of articles can lead to a number of undesirable effects including:                Attraction of dust particles.        Attraction between surfaces, e.g. plastic films and textile yams.        Risk of fire or explosion caused by sparking near inflammable liquids, gases, and explosive dusts, e.g. coal dust and flour.        Risk of shock to persons handling equipment.        
The accumulation of electrostatic charges can be prevented by using materials of low resistance. The resistivity of natural rubber can be lowered by compounding with suitable ingredients. Alternatively, as static electrification is a surface phenomenon, the product can be covered with a conducting surface layer.
Low resistance rubber is required for a wide range of applications, such as rollers for textile machinery, conveyor belting, fuel hoses, flooring, footwear, antistatic gloves (electronic industry), cables, equipment used in hospital operating theatres, and aircraft components.
The terms “antistatic” and “conductive” are restricted here to rubber products rather than the rubber itself because the electrical resistance of the product depends not only on the resistivity of the rubber but also on the shape and most probable positions of charge generation and discharge.
Natural rubber is normally considered to be an electrically insulating material but it can be an electrically insulating material but it can be compounded to give electrical resistivity lying anywhere between 1 ohm/cm. and 1015 ohm/cm. The most common means of reducing resistance is to add a suitable carbon black (super conductive furnace). Resistance falls with a decrease in particle size, increase in black “structure” and increase in concentration. For light coloured products certain grades of aluminium silicate may be used as antistatic fillers although these are usually less effective in reducing resistance than the super conductive furnace. There are also other proprietary antistatic agents that are available, such as ethylene oxide, but still these agents are less effective then the super conductive furnace.
The applicant has found that carbonised rice husk is particularly suited for use as a filler in plastics as it has been found to enhance the conductivity and flame retardant properties of the composite plastics.
Honeycomb structure of a matrix is supposed to be one of the strongest structures that have been determined by Structural Engineers. The strength comes about from the full depth hexagons and half-depth trapezoids. This type of structures is presently used as designs for building bridge decks.
The rice husk has a similar type of honeycomb design, which results in not only providing strength to the matrix, but also has sound and thermal insulation properties. The Sound insulation property is provided by the micro-cellular structures formed by the honeycomb structure in the brown rice husk. Thus the sound is trapped within the microcellular structure. This property is inherent to the brown (fresh) rice husk. The Thermal insulation property is provided by the honeycomb structure, which is strengthened by the silica and fibre which predominately present in brown rice husk and lesser in the carbonised (depend to the rate of carbonising) rice husk.
The presence of appreciable quantities of oxygen on the surface of carbonised rice husk acts as insulation for each aggregate, thereby reducing the conductivity and also reducing the flammability. The presence of nitrogen and oxygen in the fresh husk not only enables the blowing effect but also nitrogen being inert reduces the flame spread. The volume of gas (nitrogen/oxygen) evolution per gram of fresh rice husk is 240 ml/g. The husk's decomposition temperature is at about 280° C. and curing temperature of rubber and ethyl vinyl acetate is between 130° C.-180° C., thus when urea (dinitroso pentamethylene tetramine) is milled along the decomposition temperature is reduced within the curing temperatures. The presence of silica in the rice husk provides better mechanical strength.
Typical chemical and physical properties of fresh and carbonised rice husk are detailed as follows:                consists of 20-23% of paddy        husk burning: 20% ash by weight 90-95% is silica (amorphous and crystalline)        physical characteristics: bulk density 96.12-112.14 kg/m3         pH 7.14 (husk ash)        moisture content 5.6-7.2%, dry basis        ash 22.2%        
Chemical CompositionMoisture Content5.6-7.2%, dry basisAsh:22.2%Protein 2.4%Crude fat 0.7%Carbohydrate32.0%Fresh RHCarbonised RHAl2O30.025% 0.023%CaO0.36% 0.12%NaO0.034% 0.018%SiO296.2%53.88%Fe2O30.041% 0.022%MgO0.16%0.078%K2O0.69% 0.95%P2O50.57% 0.27%
It is an object according to one aspect of the present invention to provide an alternative filler which will enhance the antistatic, flame retardant, accelerator, plasticiser blowing and/or other physical or mechanical in composite materials. The filler is desirably cheap, environmentally friendly and replenishable and it does not compromise other characteristics of the composite material.